Within the last years BAM has carried out numerous drop tests with prototype casks made of ductile cast iron onto targets according to the requirements for final disposal of non-heat generating waste in the German KONRAD repository. The results have shown that the target specifications in the acceptance criteria have to be defined more accurately to get reproducible test results with high precision. Hence, a suitable test stand foundation was developed with much effort. The integrity of the upper concrete layer of this target must be preserved during a test.
Recently the geometrical properties of a tested cubic cast iron container led to a concentration of the impact forces beneath the container walls. The target was damaged strongly with the consequence of inadmissible reduction of cask stresses. For that reason the target construction was modified. However, the basic design was not changed. A prefabricated concrete slab was still joined by a mortar layer to the IAEA target of the BAM drop test facility. In the course of the optimization of the test stand foundation the concrete slab dimensions and the reinforcement were enlarged. During the drop test repetition the target kept intact. Additionally, the mechanical behavior of the cast iron container and the target was analyzed by finite element calculations.
This improved target construction is suggested as a reference target for drop tests with casks whose mass and base area are covered by the container types VI or VII respectively according to the KONRAD repository acceptance criteria.
The measurements during the drop tests with cast iron casks have provided the strains on the cask surface at selected positions. This allows the verification of finite element simulations of drop tests which show the stress distribution also inside the component. In September 2008 a drop test was carried out with a cylindrical cast iron cask containing an artificial material defect which was designed under consideration of critical stress states in the cask body. This drop test could demonstrate the safety against failure by fracture of a cask made of a special cast iron with reduced fracture toughness.
BAM is the responsible authority in Germany for the assessment of the mechanical and thermal design safety of packages for the transport of radioactive materials. The assessment has to cover the proof of brittle fracture safety for package components made of potentially brittle materials. This paper gives a survey of the regulatory and technical requirements for such an assessment according to BAM's new 'Guidelines for the application of ductile cast iron for transport and storage casks for radioactive materials'. Based on these guidelines, higher stresses than before will be permissible, but it is necessary to put more effort into the safety assessment procedure. The fundamentals of such a proof using the methods of fracture mechanics are presented. The recommended procedure takes into account the guidelines of the IAEA's advisory material which are based on the prevention of crack initiation. Examples of BAM's research and safety assessment practices are given. Recommendations for further developments towards package designs with higher acceptable stress levels will conclude the paper.
The use of high niobium alloyed cast iron alloys is a relatively new approach in which the niobium addition intends to improve the properties of the material by the precipitation of hard niobium carbides during solidification. Steels can be replaced by ductile cast iron in some rolling applications, such as gears and cams, in order to reduce material costs. The aim of this work is to evaluate ductile iron alloyed with 1 weight percent (wt.%) niobium for the as cast specimens and with 1.8 wt.% and 2.4 wt.% niobium for the austempered specimens under lubricated slip-rolling tests using mixed/boundary conditions in an Amsler-type machine. Austempered ductile iron (ADI) alloyed with 1 wt.% chromium, or Carbidic ADI, was tested for comparison. For the as cast conditions, the niobium addition resulted in an increase of wear resistance owing to the low contact pressure of these tests. However, for the austempered specimens, the best performance was found for unalloyed ADI. The main factor acting in the initiation and propagation of cracks in ductile iron is the presence of the graphite nodules. The coarse carbides also contributed to the initiation of cracks and spalling of the material.
Carbidic Austempered Ductile Iron (CADI) microstructures containing eutectic carbides can be produced by the addition of carbide stabilizing elements, such as chromium. Carbides formed from the addition of Cr are eutectic of M3C type. The presence of such hard phases can enhance the abrasion wear resistance of ductile iron. A new CADI can be produced by the addition of Nb. Niobium carbide particles are formed in the beginning of solidification and remain stable once they are insoluble in solid iron matrix. The dry sand abrasive wear resistance of ductile irons alloyed with 1.0, 1.8, and 2.4 wt% Nb were tested in both “as-cast” and “heat treated” conditions using standard ASTM G65. Results were compared to abrasive wear data obtained on ductile iron alloyed with 1 wt% Cr, CADI (1 wt% Cr), and the basic composition of iron without carbide stabilizing elements. In the “ascast” condition, the addition of Nb did not lead to a reduction in wear, while CADI with Nb is a promising substitute for CADI with Cr addition, because both materials showed very similar values of abrasion resistance.
Micro-ploughing and micro-cutting mechanisms were observed on the worn surfaces of ductile irons. Abrasive wear resistance of these alloys was correlated with the volume fraction of carbides.